possessed beyond this we know nothing; and in tracing back the springs of the origin of everything that we call "modern science" (which is not merely knowing, but knowing systematically, and with the intention and endeavour to find out the causal connection of things)—I say that when we trace back the different lines of all the modern sciences we come at length to one epoch and to one country—the epoch being about the fourth and fifth centuries before Christ, and the country being ancient Greece. It is there that we find the commencement and the root of every branch of physical science and of scientific method. If we go back to that time we have in the works attributed to Aristotle, who flourished between 300 and 400 years before Christ, a sort of encyclopaedia of the scientific knowledge of that day—and a very marvellous collection of, in many respects, accurate and precise knowledge it is. But, so far as regards this particular topic, Aristotle, it must be confessed, has not got very far beyond common knowledge. He knows a little about the structure of the heart. I do not think that his knowledge is so inaccurate as many people fancy, but it does not amount to much. A very few years after his time, however, there was a Greek philosopher, Erasistratus, who lived about three hundred years before Christ, and who must have pursued anatomy with much care, for he made the important discovery that there are membranous flaps, which are now called "valves," at the origins of the great vessels; and that there are certain other valves in the interior of the heart itself. Fig. 1.—The apparatus of the circulation, as at present known. The capillary vessels, which connect the arteries and veins, are omitted, on account of their small size. The shading of the "venous system" is given to all the vessels which contain venous blood; that of the "arterial system" to all the vessels which contain arterial blood.